Implantable medical leads having a helix, which is rotatable as well as extendable out from and retractable into a tubular header in the distal end of the lead, and which functions both as a fixation means, for attaching to the tissue of an organ inside a body, and an electrode member for transmitting and/or receiving electrical signals to and from the organ, are well known in prior art. Such medical implantable leads are e.g. used to connect a pacemaker or a cardiac defibrillator device to a heart, but also other applications for monitoring and/or controlling the function of other organs could be conceivable with such a lead.
Normally, such a medical implantable lead is implanted from the outside of the body. For example in case of a pacemaker, the lead can be introduced into a heart through a vein and attached to the inner surface of a heart wall. The physician performing the implantation cannot actually see when the implantation is performed other than by means of x-ray imaging and accordingly it is hard to verify the performance of the attachment.
It can be extremely important for a patient that the fixation of the helix is done properly, both in relation to transmitting of electrical signals between the tissue and the helix as well as in relation to long term secure attachment of the lead to the organ such that the lead is not accidently disengaged from the organ. Poor helix fixation is a well known problem and till now there has been no good way of verifying whether a helix is properly secured to the tissue or not. In prior art it has been known to e.g. perform current of injury measurements, i.e. to measure the potential difference between injured tissue, e.g. tissue which is penetrated by a helix, and uninjured tissue, or to perform mechanical pull tests of the attached lead. Both of these methods are associated with drawbacks since they can appear to indicate a proper fixation even when only a small part of the helix actually is secured to the tissue. Also, by manufacturing the header and the helix of a radiopaque material it is, by means of x-ray imaging, possible to see when a helix is sufficient screwed out from the header by counting the number of wire loops that is visible beyond the distal end of the header. However, by this method it is not possible to see whether these wire loops are embedded into tissue or not, since the tissue itself is not visible on the images. Another way of ensuring that the helix is sufficient screwed out from the header is to count the number of turns the physician is rotating a helix rotating control member when performing the screwing out of the helix from the proximal end. Due to occurring elastic properties in a rotatable torque transferring member, usually a rotatable wire coil, extending from the helix rotating control member to the shaft, and friction between the torque transferring member and the rest of the surrounding lead, it is however necessary to rotate the helix rotating control member more than the theoretically required number of turns to be sure that the helix will be sufficient screwed out. This result in a risk that the helix my become completely screwed out while the physician continuous to rotate the helix rotating control member such that the distal end of the whole lead is twisted and may perforate e.g. a heart wall.